一种新的稀有金属矿床定年技术:微区原位Sm-Nd定年  

A new age determination technique for rare metal deposits:In situ Sm-Nd dating

作　　者：李超[1,2] 孙鹏程 孟会明 侯江龙 周利敏[1,2] 赵鸿[1,2] 赵令浩 屈文俊[1,2] LI Chao;SUN PengCheng;MENG HuiMing;HOU JiangLong;ZHOU LiMin;ZHAO Hong;ZHAO LingHao;QU WenJun(National Research Center for Geoanalysis,Beijing 100037,China;Key Laboratory of Re-Os Isotope Geochemistry,China Geological Survey,Beijing 100037,China;Faculty of Land Resource Engineering,Kunming University of Science and Technology,Kunming 650093,China;Institute of Mineral Resources,Chinese Academy of Geological Sciences,Beijing 100037,China)

机构地区：[1]国家地质实验测试中心,北京100037 [2]中国地质调查局Re-Os同位素地球化学重点实验室,北京100037 [3]昆明理工大学国土资源工程学院,昆明650093 [4]中国地质科学院矿产资源研究所,北京100037

出　　处：《岩石学报》2022年第2期445-454,共10页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41873065);科技部国家重点研发计划项目(2020YFA0714800)联合资助.

摘　　要：^(147)Sm-^(143)Nd放射性同位素体系在地球科学研究中得到了广泛的应用,经典的同位素稀释-热表面电离质谱法(ID-TIMS)一直是Sm-Nd同位素高精度测定的基准技术,但具有耗时长、成本高、样品需求量大等缺点,并且难以揭示微观尺度单矿物所蕴含的地球化学信息。近年来兴起的微区原位分析,具有简单、快速、高空间分辨率的特点,可以从微米尺度示踪岩浆和热液的起源及演化过程。本文通过同时测定Sm和Nd同位素质量分馏系数,实现^(144)Sm对^(144)Nd干扰的准确校正,获得了人造玻璃、磷灰石、榍石、独居石等几种不同基体标准样品(NIST610、Durango、MAD-2、BLR-1、117531)精确的^(143)Nd/^(144)Nd比值,与推荐值在误差范围内一致。然而,由于Sm和Nd元素性质的差异,在激光剥蚀和质谱电离过程中会产生明显的元素分馏,导致^(147)Sm/^(144)Nd很难进行精确校正,本文通过在进样系统中引入液态气溶胶,有效克服了基体效应的影响,虽然硅酸盐矿物(人造玻璃、榍石)和磷酸盐矿物(磷灰石、独居石)两者之间Sm、Nd元素分馏系数差异明显,但是同一类型的不同矿物具有稳定的、一致的元素分馏系数,因此采用外标法能够获得精确的^(147)Sm/^(144)Nd,进而使得微区原位^(147)Sm-^(143)Nd定年成为了可能。采用新建立的飞秒激光-多接收等离子体联用微区原位Sm-Nd同位素分析技术(fs-LA-MC-ICPMS),获得了河北麻地稀有金属矿床中独居石、氟碳铈矿原位Sm-Nd年龄为177.2±6.3Ma,与独居石微区原位U-Pb年龄175.6±1.1Ma在误差范围内一致,证实了该方法的准确性和有效性。结果表明,独居石、氟碳铈矿原位Sm-Nd同位素分析能够作为一种有力的技术工具,不仅能够直接厘定稀有金属矿床的形成时代,同时还能够利用初始^(143)Nd/^(144)Nd比值揭示成矿物质来源的有效信息。^(147)Sm-^(143)Nd radioisotope system has been widely used in geoscience.The classical isotope dilution thermal ionization mass spectrometry(ID-TIMS)has always been the standard methods for high-precision determination of Sm-Nd isotopes.However,it is time-consuming,high cost and large demand for samples,and it is difficult to reveal the geological information especially in micro scale for a single crystal.In recent years,the in-situ analysis,which is simple,rapid and high spatial resolution,can trace the origin and evolution of ore deposits and magmas from the micro scale.In this study,the mass fractionation coefficients of Sm and Nd were determined simultaneously,the interference of 144 Sm on 144 Nd was corrected accurately.The accurately 143 Nd/144 Nd ratios of different matrix standard samples(NIST 610,Durango,MAD-2,BLR-1,117531)were obtained,which were consistent with the recommended values within the error range.However,due to the different chemical properties of Sm and Nd,there will be obviously element fractionation in the process of laser ablation and mass spectrometry ionization,which makes it difficult to accurately correct 147 sm/144 Nd.In this study,the introduction of liquid aerosols in the sample inlet system can effectively overcome the influence of matrix effect.There are distinctly differences in Sm and Nd elemental fractionation coefficients between silicate minerals(artificial glass,sphene)and phosphate minerals(apatite,monazite,etc.),but the same type of minerals has stable and consistent elemental fractionation coefficients.Therefore,the external standard method can obtain accurate 147 Sm/144 Nd,which makes it possible for in-situ dating of 147 Sm-143 Nd.In situ Sm-Nd isotopic dating of monazite and bastnaesite in the Madi rare earth deposit,Hebei Province,has been carried out by using the newly established fs-LA-MC-ICPMS methods.It’s show that the age of monazite and bastnaesite is 177.2±6.3Ma,which is consistent with the in situ monazite U-Pb age of 175.6±1.1Ma.Those indicate that in-si

关 键 词：SM-ND同位素 LA-MC-ICPMS 定年 独居石 氟碳铈矿 稀有金属矿床 

分 类 号：P597[天文地球—地球化学] P618.6[天文地球—地质学]